1. Field of the Invention
This invention relates to electric furnaces for heating molten glass and their methods of operation and more particularly to the interconnection of current source outputs for increasing electric power available for Joule
2. Description of the Prior Art
In the manufacturing of glass, an electric furnace may be utilized to melt a batch of raw materials in a refractory lined furnace chamber. Although hydrocarbon fuel burning furnaces may also be utilized to produce glass, the electric furnace has certain advantages with respect to the problems of air pollution and maintenance of uniform heating.
Typically, an electric furnace will have two or more electrodes submerged in the molten glass which are connected to a source of alternating current. The resistivity of the molten glass transfers the electrical energy of the current flowing between electrodes into heat energy thereby creasing Joule effect heating. Molten glass has a negative temperature coefficient and therefore, the resistivity below a critical temperature is sufficiently high so as to limit current flow below a level at which electric melting can be sustained. The power supplied to the furnace chamber can be regulated by phase controlling the applied voltage with suitable means, typically silicon controlled rectifiers. Since, during normal operation, the phase control is operating at 92% to 95% of the voltage cycle to obtain a favorable power factor and the power supplies and phase controls are operated at or near their ratings, neither the current flow nor the voltage can be increased significantly to raise the temperature of the molten glass. Thus, for example, when the molten glass falls below the critical temperature, its molten state has not been maintained electrically. Therefore, electric furnaces generally require a plurality of fuel burners positioned to direct radiant heat to the upper surface of the material in the furnace chamber. This radiant heat melts the material until the critical temperature is reached above which the resistivity of the molten glass is low enough to permit sufficient current to flow between the electrodes for normal controlled electric heating furnace operation.
Glass which is utilized in the production of glass wool often has alkali metals, such as sodium or potassium, added as a flux to facilitate melting of the batch material and to lower the viscosity of the molten glass to decrease production time. These alkali metals also cause the molten glass to have low resistivity which aids the melting process in an electric furnace. However, some glasses, typically those utilized for the manufacture of continuous filaments, generally referred to as "E" glass, have less than 1% alkali metal content and therefore exhibit relatively high resistivity as compared to the wool glass, for example, 10 to 12 times that of wool glass even at melting and refining temperatures. Further, a wool glass may normally be refined at about 2500.degree.F and for a given set of parameters for electrical melting reaches a critical temperature below which electrical melting retrogresses at about 2300.degree.F while E glass will be refined to about 2600.degree.F and have a critical temperature of about 2400.degree.F for those parameters.
However, the use of fuel burners creates undesirable combustion products and emissions from the batch material. Fluorine is often added to E glass as a flux to aid in placing some of the components of the batch materials in solution, to reduce bubbles in the molten glass and to reduce the viscosity of the molten glass. During the melting and refining process much of this fluorine is driven off with boron and other elements which may also be included in the batch material. In order to militate against these factors, glass melting and refining is performed in electric furnaces employing a cold top wherein a layer of batch material covers substantially the entire upper surface of the molten glass and batch material is added to the upper surface as the lower surface of the batch layer is melted. However, if the electric furnace is to be restarted after the power has been interrupted for a period of time sufficient to allow the molten glass to cool below the critical temperature so that it has a relatively high resistivity, in the past it has been necessary to apply radiant heat to melt the cold top crust and the underlying molten glass to lower the resistivity to permit sufficient current flow for normal furnace operation. During this restart period, the undesirable emissions from the batch material and the products of combustion are generated.
An object of this invention is to facilitate the electric heating of molten glass.
A second object is to increase rapidly the temperature of molten glass which is heated electrically.
A third object is to avoid, during the campaign of a glass tank in which molten glass is heated electrically, the application of heat to the top of a mass of glass constituents.
A fourth object is to expand the range of molten glass temperature over which electric heating is effective to raise the molten glass to suitable melting, refining and working temperatures.
A fifth object is to enable the temperature of molten glass to be raised to the critical temperature for the electrical heating and tank parameters of the system at which normal electrical heating will increase the glass temperature.
Another object is to increase Joule effect heating in selected localized regions of molten glass.